Improved the section header titles in the API Reference chapter.
Ensured that all generated man pages have an initial Library section.
Added manual index entries for all public header files.
Added manual index entries for various chapters and topics.
Defined a Glossary chapter for the manual.
Added currentmodule directives to all public symbol documentation.
Ensured that Sphinx generates a module index for the manual.
Defined a placeholder for a custom 'c_header' Sphinx extension.
Imported a Doxygen configuration file template generated with:

    $ doxygen -g doc/doxygen/Doxyfile
Customized the Doxygen configuration file.
Added Doxygen @file annotations to all header files.
Added placeholders for example code snippets in the Doxygen output.
Added Doxygen descriptions for all CPR_ABBREV-dependent aliases.
Added a main page placeholder for the Doxygen documentation.
Fixed some Doxygen cross-references to global variables and typedefs.
Implemented and documented the remaining cpr_*_alloca() macros.
Documented the preconditions for the cpr_vector_*() functions.
Defined @error to be an alias for @exception in Doxygen input.
Ensured that cpr_vector_push_back() checks all preconditions.
Ensured that cpr_vector_reserve() catches std::bad_alloc as well.